A conventional pneumatic ratchet wrench is operated in a single direction and contains a reducing gear set driven by an air motor of a single chamber cylinder so as to reduce an output rotating speed and to increase a torque, and then a crank shaft drives a ratchet yoke so as to actuate a drive square to rotate, wherein a pawl is shifted by a user to engage with the ratchet yoke, such that the drive square is controlled to rotate in a forward direction or a reverse direction. However, when the drive square of the conventional pneumatic ratchet wrench tighten a fastener (such as a screw or a bolt), the user has to resist a reaction force, thus causing the operation fatigue or damage.
With reference to FIGS. 1 and 2, another conventional pneumatic ratchet wrench is operation in a single direction and is driven by an air motor 11 of a single expansion chamber so as to actuate the impact clutch 12, such that a crank shaft 13 is driven by the impact clutch 12 to actuate a drive square 14 to rotate, wherein the air motor 11 has a cylinder 111, a single chamber 112 defined in the cylinder 111, two exhaust ports 113 formed on one side of the cylinder 111 and communicating with the single chamber 112, an inlet 114 arranged on one end of the cylinder 111 and connecting with the single chamber 112, and a rotor 115. Thereby, the rotor 115 drives the impact clutch 12 to actuate the crank shaft 13 so that the reaction force of the drive square 14 will not pass back to the user. But the air motor 11 rotates at a high speed, so the impact clutch 12 hits the crank shaft 13 in high frequency to shorten tool service life.
U.S. Pat. No. 7,080,578 discloses that a pneumatic ratchet wrench is driven by an air motor via a reduction gear set so as to actuate the impact clutch to operate in a low speed, and then the impact clutch drives a crank shaft to rotate a drive square. Yet such a pneumatic ratchet wrench includes the reduction gear set, thus having complicated structure and increasing the tool size, weight, and production cost.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages.